Check the appropriate box to designate the rule pursuant to which this Schedule is filed:

☒ Rule 13d-1(b)

☐
Rule 13d-1(c)

☐ Rule 13d-1(d)

*

The remainder of this cover page shall be filled out for a reporting persons initial filing on this form with respect to the subject class of securities, and for any subsequent amendment containing information
which would alter the disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be
deemed to be filed for the purpose of Section 18 of the Securities Exchange Act of 1934 (Act) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act
(however, see the Notes).

PAGE 1 OF 4 PAGES

1

NAME OF
REPORTING PERSON

S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON

NWQ Investment Management Company, LLC
47-0875103

2

CHECK THE APPROPRIATE BOX IF A MEMBER
OF A GROUP*

(a) ☐ (b) ☐

N/A

3

SEC USE ONLY

4

CITIZENSHIP OR PLACE OF
ORGANIZATION

Delaware - U.S.A.

NUMBER OF

SHARES

BENEFICIALLY

OWNED BY

EACH

REPORTING

PERSON

WITH

5

SOLE VOTING POWER

6,034,466

6

SHARED VOTING POWER

0

7

SOLE DISPOSITIVE POWER

6,034,466

8

SHARED DISPOSITIVE POWER

0

9

AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

6,034,466

10

CHECK BOX IF THE AGGREGATE AMOUNT IN
ROW (9) EXCLUDES CERTAIN SHARES*

N/A

11

PERCENT OF CLASS REPRESENTED BY AMOUNT
IN ROW 9

4.987%

12

TYPE OF REPORTING PERSON*

IA

PAGE 2 OF 4 PAGES

Item 1(a)

Name of Issuer:

Mitel Networks Corporation

Item 1(b)

Address of Issuers Principal Executive Offices:

350 Legget Drive

Kanata

Ottawa ON K2K 2W7

Canada

Item 2(a)

Name of Person Filing:

NWQ Investment Management Company, LLC

Item 2(b)

Address of the Principal Office or, if none, Residence:

2049 Century Park East, 16th
Floor

Los Angeles, CA 90067

Item 2(c)

Citizenship:

Delaware  U.S.A.

Item 2(d)

Title of Class of Securities:

Common

Item 2(e)

CUSIP Number:

60671Q104

Item 3

If the Statement is being filed pursuant to Rule 13d-1(b), or 13d-2(b), check whether the person filing is a:

(e)

☒ An investment advisor in accordance with section 240.13d-1(b)(1)(ii)(E)

Item 4

Ownership:

(a)

Amount Beneficially Owned:

6,034,466

(b)

Percent of Class:

4.987%

(c)

Number of shares as to which such person has:

(i)

sole power to vote or direct the vote:

6,034,466

(ii)

shared power to vote or direct the vote:

0

(iii)

sole power to dispose or to direct the disposition of:

6,034,466

(iv)

shared power to dispose or to direct the disposition of:

0

Item 5

Ownership of Five Percent or Less of a Class:

If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities, check the
following ☒.

PAGE 3 OF 4 PAGES

Item 6

Ownership of More than Five Percent on Behalf of Another Person:

Not applicable.

Item 7

Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on By the Parent Holding Company:

Not applicable.

Item 8

Identification and Classification of Members of the Group:

Not applicable.

Item 9

Notice of Dissolution of a Group:

Not applicable.

Item 10

Certification:

By signing below I certify that, to the best of my knowledge and belief, the securities referred to above were acquired and are held in the ordinary course of business and were not acquired and are not held for the purpose of or
with the effect of changing or influencing the control of the issuer of such securities and were were not acquired in connection with or as a participant in any transaction having such purpose or effect.

SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true,
complete and correct.